Referring to FIG. 1, a conventional concrete screw 1 includes a head 11 and a shank 12 integrally extending from the head 11, a first thread 13 helically extending on the shank 12, and a second thread 14 helically extending on the shank 12 and alternating with the first thread 13. The first thread 13 has an outer diameter larger than that of the second thread 14, and each of the first and second threads 13, 14 tapers off towards an outer circumference thereof. When the screw 1 is tightened to a concrete object, the first and second threads 13, 14 cut and ream the concrete material to be readily screwed-in the concrete object. However, because of the sharp edges of the threads 13, 14, the screw is liable to slant and wobble during a starting screw-in movement of the screw 1 from a pilot end 121 of the shank 12. In a slanted state, the screw 1 is subject to a relatively large and increased frictional force during the screw-in movement, and a relatively large torque is required to overcome the frictional force. Thus, the screw-in operation is effortful and time-consuming, and the screw 1 tightened on the concrete object is slanted.
Referring to FIG. 2, another conventional concrete screw 2 includes a head 21, a shank 22 integrally extending from the head 21, and first, second and third threads 23, 24, 25 which helically extend on the shank 22 and which have different and decreased outer diameters. The first thread 23 includes a plurality of helically extending thread segments 231 intermittent to each other by a plurality of non-thread segments 232. When the screw 2 is tightened to a concrete object, the first, second and third threads 23, 24, 25 with different and decreased outer diameters guide the screw-in movement of the screw 2. The drawback of the screw 2 resides in that the thread segment 231 of the first thread 23 near the pilot end 221 of the shank 22 has a sharp edge and thus renders the screw 2 to slant during a starting screw-in movement. Moreover, a space provided by the non-thread segments 232 is not sufficient to accommodate swarf generated during the screw-in movement. The swarf may interfere with the screw-in movement and may result in an effortful and time-consuming operation.